Image dissector tube field mesh



July 4, 1967 R. H. FOOTE 3,329,856

IMAGE DISSECTOR TUBE FIELD MESH I Q 7 Filed Sept. 24, 1964 ELLE-Emmi 323; 36 9 l s. 1* JLIS BY I ATTORNEYS United States Patent M 3,329,856IMAGE DISSECTOR TUBE FIELD MESH Richard H. Foote, Fort Wayne, Ind.,assignor to International Telephone and Telegraph Corporation, Nutley,N.J., a corporation of Maryland Filed Sept. 24, 1964, Ser. No. 398,891 7Claims. (Cl. 315-11) ABSTRACT OF THE DISCLOSURE A curved field meshaccelerator in an image dissector tube provides focusing for electronsfrom all areas of a planar photocathode directed toward a scanningaperture.

This invention relates generally to image dissector tubes, and moreparticularly to an image dissector tube in which the need for dynamicfocusing is eliminated.

A common type of image dissector tube comprises an extended area, planarphotocathode which provides a low velocity flood beam of electronsmodulated in accordance with excitation of the photocathode by anoptical image. The modulated flood beam is scanned over a small definingaperture so that the electrons which pass through the aperture at anyinstant emanate from only a single incremental area of the photocathode.The electrons which pass through the aperture are conventionallymultiplied by a secondary electron multiplier to provide a time-basedvideo output signal corresponding to the electron image which has beenscanned over the aperture.

Since the transit time of electrons from the axial center of thephotocathode to the defining aperture is normally less than the transittime of electrons emanating from the outer edge of the photocathode,conventional image dissector tubes have employed various dynamicfocusing systems in an effort to achieve optimum focusing of electronsemanating from the entire area of the photocathode and thus to provideoptimum resolution. Such dynamic focusing systems, however, requireexternal circuitry for generating and applying the dynamic focusingwaveforms thus increasing the complexity and in turn, the cost of thesystem in which the tube is employed.

It is accordingly an object of the invention to provide an imagedissector tube in which optimum focusing of the electron beam isprovided without the use of a dynamic focusing system.

The invention in its broader aspects provides an image dissector tubehaving a source of an extended area electron beam with first electrodemeans spaced axially from the beam source and having a defining aperturetherein and with secondary electron multiplier means being provided forreceiving and multiplying the electrons of the beam which pass throughthe aperture. A screen electrode is provided disposed between the beamsource and the first electrode and means are provided acting upon thebeam between the screen electrode and the first electrode for scanningthe beam over the aperture. The beam is focused by means of an axialmagnetic field extending through the tube and a suitable potential isapplied to the screen electrode so that essentially all of theacceleration of the beam electrons occurs between the beam source andthe electrode. In order to insure that the transit times of theelectrons of the beam from all points on the beam source to the apertureare substantially equal to the times required for the electron to makean integral number of complete revolutions in the magnetic field, thescreen electrode is curved toward the first electrode.

The above mentioned and other features and objects of this invention andthe manner of attaining them will 3,329,856 Patented July 4, 1967 becomemore apparent and the invention itself will be best understood byreference to the following description of an embodiment of the inventiontaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional view showing the improved imagedissector tube of the invention; and

FIG. 2 is a diagram useful in explaining the invention.

Referring now to FIG. 1, the improved image dissector tube of theinvention, generally indicated at 10, comprises a conventional elongatedevacuated envelope 11 having a longitudinal axis 12, opposite ends 13,14 and a side wall 15. A conventional photocathode 16 is deposited orotherwise positioned on the inner surface of the end wall 13 which istransparent to the wave length of the optical image 17. The opticalimage 17 is thus impressed upon the photocathode 16 which thus emits alow velocity flood beam of electrons area-modulated in accordance withthe optical image 17; photocathode 16 thus emits an electron imagecorresponding to the optical image 17. Photocathode 16 is coupled toterminal 18 adapted to be connected to a suitable source of potential.

Electrode 19 is axially spaced from the photocathode 16 and has adefining aperture 20 formed therein concentric with the axis 12.Electrode 19 is connected to terminal 22 adapted to be connected to asuitable source of potential. A conventional secondary electronmultiplier assembly 23 is disposed between electrode 19 and end wall 14of the envelope 11 for receiving and multiplymg the electrons of thebeam which pass through the defining aperture 20*. Electron multiplier23 comprises a plurality of dynode stages 24-31 respectively connectedto terminals 32-39 adapted to be connected to sources of progressivelyhigher potentials. A final target electrode 40 is provided connected toterminals 42 which is adapted to be connected to a suitable source ofpotential and an output electrode 43 is positioned between targetelectrode 40 and the final dynode stage 31. Target electrode 43 iscoupled by load resistor 44 to terminal 45 adapted to be connected to asuitable source of potential and is also connected to output terminal 46by coupling capacitor 47.

In order to provide for initial acceleration of the beam electrons andalso to provide an essentially field-free space within the envelope fordeflection of the electron beam, a field screen electrode or mesh 48 isprovided extending across envelope 11 between photocathode 16 andelectrode 19 and closely spaced from the photocathode 16, as shown.Field mesh 48 is connected to terminal 49 adapted to be connected to asuitable source of accelerating potential. A tubular electrode 50 isprovided extending axially substantially the entire distance between thefield mesh electrode 48 and electrode 19. Electrode 50 may be formed asa conventional conductive coating on the interior-surface of side wall15 of the envelope 11 and may be connected to the field mesh electrode48- and the electrode 19, or may be electrically isolated therefrom, asshown. Tubular electrode 50 is formed of nonmagnetic material in orderto permit magnetic deflection of the electron beam. Tubular electrode 50is connected to terminal 52 adapted to be connected to a suitable sourceof potential. Terminals 49, '52 and 22 may be connected to the samepotential, or terminals 52 and 22 may be connected to a source ofpotential a few volts higher than that connected to terminal 49, eitherconnection establishing an essentially field-free space within envelope11 between the screen electrode 48 and the electrode 19 to permitmagnetic deflection of the electron beam.

Conventional vertical and horizontal magnetic deflection yokes 53, 54are provided on the exterior of the side wall 15 of envelope 11 and thusact upon the electron beam within the tubular electrode 50 to scan thesame over the defining aperture 20 in the electrode 19.

In order to focus the electrons of the beam onto the plane of thedefining aperture 20, a solenoid coil 55 is provided coaxial with axis12 and surrounding side wall 15 of the envelope 11 and the deflectioncoils 53, 54. Solenoid coil 55, when suitably energized provides asolenoidal magnetic field extending axially through the envelope 11parallel with the axis 12.

Proper focusing of the electron beam onto the plane of the aperture 20is obtained when the photocathode 16 is spaced from the plane of theaperture 20 by an integral number of full loops of focus, i.e., when thetransit time of the electrons from the photocathode 16 to the plane ofthe aperture 20 is equal to the time required for an electron to make anintegral number of complete revolu tions in the magnetic field providedby the solenoid focusing coil 55.

Referring now additionally to FIG. 2, L is the distance between thephotocathode 16 and the plane of the aperture 20, S is the distancealong the axis 12 between photocathode 16 and screen electrode 48, 56 isa point on the screen electrode 48 radially spaced from axis 12, b isthe distance between point 56 and aperture 20, i.e., the line of travelof a beam electron from point 56 to aperture 20, a is the distance fromthe point 56 on screen electrode 48 to the photocathode 16 formed as anextension of the line of travel of the electron, and S is the distancefrom photocathode 16 to point 56 on screen electrode 48 parallel withthe axis 12. Proper focus is obtained when the transit time I of anelectron from the photocathode 16 to the aperture 20 is equal to thetime required for the electron to make an integral number of completerevolutions in the magnetic field provided by the solenoid coil 55.Thus:

In the case of an electron emanating from point 58 on the photocathode16 and thus traveling along axis 12:

so ;9 ifi w V/2 V V 2) It will be seen, however, that electronsemanating from point 59 on photocathode 16 or from any other pointthereon radially spaced from axis 12, when deflected to the aperture 20by the scanning field provided by deflection coils 53, 54 move with ahigher angular velocity. Thus:

Therefore, in order to provide the proper transit time for all electronsdeflected to the aperture 20 without regard to their point of origin onthe photocathode 16, the transit time of the off-axis electrons must bedecreased so that:

From the geometry shown in FIG. 2, it will be seen that:

COS or Solving (7) and (8) for a,

a=(L+S cos a- COS a (L+S cos aL cos a (9) Thus, it is seen that theseparation S between the photocathode 16 and the field screen electrode48 at any point radially spaced from the axis 12 is:

S=(L+S Cos aL (10) In a specific example of a tube incorporating theinvention in which the distance L is 6.25 inches and the distance S is0.25 inch, the following values for S defining the curvature of thescreen electrode 48 toward the aperture 20 are calculated in accordancewith the Equation 10 above:

at S 1 0.2474 2 0.2422 3 0.2318 4 0.2188 5 0.2006

The foregoing specific example in which a focusing field of 40 gauss isprovided, the following potentials may be applied:

Volts While I have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention.

What is claimed is:

1. An image dissector tube comprising: a source of an extended areaelectron beam; first electrode means spaced axially from said beamsource and having a defining aperture therein; secondary electronmultiplier means for receiving and multiplying the electrons of saidbeam which pass through said aperture; a screen electrode between saidbeam source and first electrode; means acting upon said beam betweensaid screen electrode and first electrode for scanning said beam oversaid aperture; and means for providing a magnetic field extendingaxially through said tube for focusing said beam onto the plane of saidaperture; said screen electrode being curved toward said first electrodeso that the transit times of the electrons of said beam from all pointson said beam source to said aperture are substantially equal to thetimes required for said electrons to make an integral number of completerevolutions in said magnetic field.

2. An image dissector tube comprising: an essentially planar source ofan extended area electron beam, said beam source being normal to anaxis; first electrode means spaced axially from said beam source andhaving a defining aperture therein on said axis; secondary electronmultiplier means for receiving and multiplying the electrons of saidbeam which pass through said aperture; an extended area screen electrodeextending across said axis between and facing said beam source and firstelectrode; means acting upon said beam betweeen said screen electrodeand said first electrode for scanning said beam over said aperture; andmeans for providing a magnetic field extending parallel with said axisthrough said tube for focusing said beam onto the plane of saidaperture; said screen electrode being smoothly curved toward said firstelectrode with its point on said axis being closer to said firstelectrode than points radially spaced from said axis so that the transittimes of electrons of said beam from all points on said beam sources tosaid aperture are substantially equal to the times required for saidelectrons to make an integral number of complete revolutions in saidmagnetic field.

3. The tube of claim 2 wherein the curvature of said screen electroderesponds to the equation:

S: (L-l-S cos OLL where:

L=the distance along said axis between said beam source and saidaperture,

S =the distance along said axis from said beam source to said screenelectrode,

Ot=th6 angle defined by said axis with the line of travel of a beamelectron between one point on said screen radially spaced from said axisand said aperture,

S=the distance along a line parallel with said axis between said beamsource and said one point on said screen electrode.

4. An image dissector tube comprising: an essentially planar source ofan extended area electron beam, said source being normal to andconcentric with an axis, first electrode means spaced axially from saidsource and having a defining aperture therein on said axis; secondaryelectron multiplier means for receiving said muliplying the electrons ofsaid beam which pass through said aperture; an extended area screenelectrode extending across and concentric with said axis between andfacing said beam source and said first electrode; a tubular electrodeconcentric with said axis between said screen electrode and said firstelectrode; means for applying potentials to said screen, tubular andfirst electrodes thereby to accelerate the electrons of said beambetween said beam source and said screen electrode and to provide anessentially field-free space between said screen electrode and saidfirst electrode; magnetic deflection means exterior to said tubularelectrode and acting upon said beam therein for scanning said beam oversaid aperture; and means for providnig a solenoidal magnetic fieldextending parallel with said axis through said tube for focusing saidbeam onto the plane of said aperture; said screen electrode beingsmoothly curved toward said first electrode with its point on said axisbeing closer to said first electrode than points radially spaced fromsaid axis so that the transit times of electrons of said beam from allpoints on said beam source to said aperture are substantially equal tothe times required for said electrons to make an integral number ofcomplete revolutions in said magnetic field.

5. The tube of claim 4 wherein said beam source is axially spaced fromthe plane of said aperture by an integral number of full loops of focusof the electrons of said beam.

6. An image dissector tube comprising: an evacuated envelope having alongitudinal axis; an essentially planar source of an extended areaelectron beam in said envelope, said source being normal to andconcentric with said axis; first electrode means in said envelope spacedaxially from said source and having a defining aperture thereinconcentric with said axis; secondary electron multiplier means in saidenvelope for receiving and multiplying the electrons of said beam whichpass through said aperture; an extended area screen electrode in saidenvelope extending across said axis and concentric therewith, saidscreen electrode being intermediate said source and said first electrodeand closely spaced from said source; a tubular electrode in saidenvelope concentric with said axis and extending substantially betweensaid screen electrode and said first electrode; means for applyingpotentials to said screen, tubular and first electrodes thereby toaccelerate said beam between said source and said screen electrode andto provide an essentially field-free space between said screen electrodeand said first electrode; magnetic deflection means exterior to saidenvelope and acting upon said beam within said tubular electrode forscanning said beam over said aperture; and a solenoid coil concentricwith said axis and surrounding said envelope and said deflection meansfor providing a solenoidal magnetic field extending axially through saidenvelope for focusing said beam onto the plane of said aperture; saidscreen electrode being convexly curved toward said first electrode withits point on said axis being axially closer to said first electrode thanpoints spaced radially from said axis so that the transit times ofelectrons of said beam from all points on said beam source to saidaperture are substantially equal to the times required for saidelectrons to make an integral number of complete revolutions in saidmagnetic field; said beam source being axially spaced from the plane ofsaid aperture by an integral number of full loops of focus of theelectrons of said beam.

7. The tube of claim 6 wherein the curvature of said screen electroderesponds to the equation:

S=(L+S C082 ocL where:

References Cited UNITED STATES PATENTS 3,207,997 3,286,114 11/1966Schlesinger 3l383 JOHN W. CALDWELL, Primary Examiner.

T. A. GALLAGHER, Assistant Examiner.

9/1965 Eberhardt 315l1 X

1. AN IMAGE DISSECTOR TUBE COMPRISING: A SOURCE OF AN EXTENDED AREAELECTRON BEAM; FIRST ELECTRODE MEANS SPACED AXIALLY FROM SAID BEAMSOURCE AND HAVING A DEFINING APERTURE THEREIN; SECONDARY ELECTRONMULTIPLIER MEANS FOR RECEIVING AND MULTIPLYING THE ELECTRONS OF SAIDBEAM WHICH PASS THROUGH SAID APERTURE; A SCREEN ELECTRODE BETWEEN SAIDBEAM SOURCE AND FIRST ELECTRODE; MEANS ACTING UPON SAID BEAM BETWEENSAID SCREEN ELECTRODE AND FIRST ELECTRODE FOR SCANNING SAID BEAM OVERSAID APERTURE; AND MEANS FOR PROVIDING A MAGNETIC FIELD EXTENDINGAXIALLY THROUGH SAID TUBE FOR FOCUSING SAID BEAM ONTO THE PLANE OF SAIDAPERTURE; SAID SCREEN ELECTRODE BEING CURVED TOWARD SAID FIRST ELECTRODESO THAT THE TRANSIT TIMES OF THE ELECTRONS OF SAID BEAM FROM ALL POINTSON SAID B EAM SOURCE TO SAID APERTURE ARE SUBSTANTIALLY EQUAL TO THETIMES REQUIRED FOR SAID ELECTRONS TO MAKE AN INTEGRAL NUMBER OF COMPLETEREVOLUTIONS IN SAID MAGNETIC FIELD.